SEC Form SC 13G filed by Upstream Bio Inc.

UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, DC 20549

SCHEDULE 13G

(Rule 13d-102)

Information to be Included in Statements Filed Pursuant

to § 240.13d-1(b), (c) and (d) and Amendments Thereto Filed

Pursuant to § 240.13d-2

Under the Securities Exchange Act of 1934

(Amendment No. )

UPSTREAM BIO, INC.

(Name of Issuer)

Common Stock, $0.001

par value per share

(Title of Class of Securities)

91678A107

(CUSIP Number)

October 15, 2024

(Date of Event Which Requires Filing of This Statement)

Check the appropriate box to designate the rule pursuant to which this Schedule is filed:

☐ Rule 13d-1(b)

☒ Rule 13d-1(c)

☐ Rule 13d-1(d)

The information required in the remainder of this cover page shall not be deemed to be “filed” for the purpose of Section 18 of the Securities Exchange Act of 1934, as amended (“Act”), or otherwise subject to the liabilities of that section of the Act but shall be subject to all other provisions of the Act (however, see the Notes).

The name of the issuer to which this filing on Schedule 13G relates is Upstream Bio, Inc. (the “Issuer”).

The principal executive offices of the Issuer are located at 890 Winter Street, Suite 200, Waltham, Massachusetts 02451.

This Schedule 13G is being filed by BCLS Fund III Investments, LP, a Delaware limited partnership (the “Reporting Person”).

Bain Capital Life Sciences Investors, LLC, a Delaware limited liability company (“BCLSI”), is the manager of Bain Capital Life Sciences III General Partner, LLC, a Delaware limited liability company (“BCLS III GP”), which is the general partner of Bain Capital Life Sciences Fund III, L.P., a Delaware limited partnership (“BCLS Fund III”), which is the managing member of BCLS Fund III Investments GP, LLC, a Delaware limited liability company (together with the Reporting Person, BCLSI, BCLS III GP and BCLS Fund III, the “Bain Capital Life Sciences Entities”), which is the general partner of the Reporting Person. As a result, BCLSI may be deemed to share voting and dispositive power with respect to the securities held by the Reporting Person.

The principal business address for each of the Bain Capital Life Sciences Entities is 200 Clarendon Street, Boston, Massachusetts 02116.

Each of the Bain Capital Life Sciences Entities is organized under the laws of the State of Delaware.

The class of securities of the Issuer to which this Schedule 13G relates is Common Stock, $0.001 par value per share (“Common Stock”).

The CUSIP number of the Common Stock is 91678A107.

If filing as a non-U.S. institution in accordance with § 240.13d-1(b)(1)(ii)(J), please specify the type of institution:

(a) Amount beneficially owned:

As of October 15, 2024, the Reporting Person held 2,295,322 shares of Common Stock, representing 4.3% of the outstanding shares of Common Stock.

The percentage of the outstanding shares of Common Stock held by the Reporting Person is based on 53,591,695 shares of Common Stock issued and outstanding following the closing of the Issuer’s initial public offering (inclusive of the exercise in full of the underwriters option to purchase additional shares), as set forth in the prospectus filed by the Issuer with the Securities and Exchange Commission pursuant to Rule 424(b) promulgated under the Securities Act of 1933, as amended, on October 11, 2024.

(b) Percent of class:

See Item 4(a) hereof.

(c) Number of shares as to which the person has:

(i) Sole power to vote or to direct the vote:

0 shares of Common Stock

(ii) Shared power to vote or to direct the vote:

2,295,322 shares of Common Stock

(iii) Sole power to dispose or to direct the disposition of:

0 shares of Common Stock

(iv) Shared power to dispose or to direct the disposition of:

2,295,322 shares of Common Stock

If this statement is being filed to report the fact that as of the date hereof the Reporting Person ceased to be the beneficial owner of more than five percent of the class of securities, check the following: ☒

Not applicable.

Not applicable.

Not applicable.

Not applicable.

By signing below I certify that, to the best of my knowledge and belief, the securities referred to above were not acquired and are not held for the purpose of or with the effect of changing or influencing the control of the Issuer of the securities and were not acquired and are not held in connection with or as a participant in any transaction having that purpose or effect, other than activities solely in connection with a nomination under § 240.14a-11.

SIGNATURES

After reasonable inquiry and to the best of its knowledge and belief, the undersigned certifies that the information in this statement is true, complete and correct.